Problems are frequently encountered during and after seating of the gate of a lined gate valve, because the plastic seat, being compressible, is deformed unsymmetrically or excessively. This means that the gate will not form an effective seal with its seat, or the seat may be damaged, or both. It is also desirable that the plastic seat have long wearing life, but while making the seat very thick might solve this problem, we have found that mere thickness will not solve the problem of unsymmetrical distortion and thus leakage at substantial pressures.